1. Field of the Invention
The present invention relates to a system to detect cracks and variations of thickness in pipelines. It relates especially to a method of detection implemented in this system. The pipelines considered herein are cylindrical and are made of an alloy or material that contains a significant proportion of ferromagnetic elements. They are mainly pipelines made of steel or iron.
Oil pipelines have to be regularly inspected for the appearance of cracks, arising partly out of corrosion or fatigue, in order to avert the possible risk of their breaking. The incipient cracks may be very small, have all sorts of shapes and be oriented in different directions. The main problem with this kind of inspection is that it is necessarily performed on site and from inside the pipelines, in portions where these pipelines are buried.
2. Description of the Related Art
Apart from ultrasonic and X-ray detection systems, there presently exists a system for the magnetic detection of cracks in a pipeline comprising a traction module and a magnetic detection module. The traction module is provided with peripheral joints and is driven by the flow of liquid flowing in the pipeline. The module may contain electronic processing circuitry, electrical power storage means and recording equipment. The magnetic detection module comprises an axial permanent magnet or magnets and, at each of its ends, brushes made of a ferromagnetic material that set up a magnetic path for the magnetic flux generated by the permanent magnets which thus gets closed in through a portion of the pipeline. The magnetic detection module is provided on its periphery with a set of Hall detectors or other detectors providing measurement signals that represent the magnetic induction on the inner surface of the pipeline. These measurement signals, which are acquired permanently during the movement of the detection system inside the pipeline, are used to obtain a magnetic signature of this pipeline.
This detection system, provided with a sequence of articulated modules and an odometer device, is inserted into a pipeline from an entry point called a siding. The pipeline which has been drained beforehand to enable the opening of the circuit and the introduction of the system is then put back into operation with a circulation of fluid. The detection of the weld joints located at predetermined distances associated with the odometry measurement enables the detected variations of thickness and cracks to be localized.
This system is used to detect cracks whose main orientation is substantially perpendicular to the axis of the pipeline. However, this system cannot be used to detect cracks that are oriented in a direction almost parallel to the axis of the pipeline and are shallow. For, the variation in the swept flux caused by this type of crack is generally far too small to be detected.